rcd

I'll have a go, but don't know if these are the answers they are after...

1. Earth fault loop impedance too high to be sure fault current protective device will operate within the required time (or even at all) on a short circuit to earth (EEBAD).
2. Protection of an IT system against earth faults (where earth fault loop impedance is deliberately high).

IIRC, the standard answer is:

Protection against: indirect contact, fire, nuisance trips.

Typically fitted where there is little or no risk of accidental direct contact and hence no need for protection against electric shock - but there is a high risk of fire, particularly in the event of malfunction. Or where the nature of the load can produce high transient imbalance currents on state change.

Thus, fitted to protect fixed, high level stage and theatre lighting, agricultural buildings, industrial buildings with remote operation of plant.

ISTR that they are also used to protect long service cables where there is a risk of fire.

I may be wrong on the above, but, hopefully, at least it may give you directions to look. I get mixed up between the rules for 300mA and 500mA devices....and would look it up, if I had to actually use one.